Is Z-Render Pass accuracy dependent on sample count? Are others like object id? (Cycles)
No, Z pass, ObjectID and alike are not dependent on Cycles render samples. Their visual output will always be the same like rendered with 1 sample - so not nice.

It's better to obtain them in Cycles with a different method - Z pass with Mist pass or Ray-depth shaders, ID-masks with random-colored shadeless materials for example.

Or get them from Blender Internal (you want to use Full Sample there).

Here is a little table what is (green) and is not (red) dependent on render samples: